Piezoelectric elements are known to include various types and shapes of devices produced from various piezoelectric materials. Typical piezoelectric elements consist of substantially rectangular or round plates made from quartz. These piezoelectric elements are commonly used for frequency control in electronic devices such as, computers, cellular phones, pagers, radios and wireless data devices, etc. As consumer demand continually drives down the size and cost of this equipment, the need for piezoelectric devices to be smaller, lower cost and automatable has become even greater.
Automated mounting equipment typically requires that each piezoelectric element, as presented to the mounting equipment, be uniform in size and shape. Typically, this equipment can include a robotic machine with a vision system, a pick-and-place system, and the like. As the size of the devices to be mounted shrink, automation systems require tighter and tighter tolerances from the elements to be mounted in order to reduce breakage, misplacement and jamming.
In FIG. 1, prior art photolithographically produced piezoelectric elements 10 are shown before being singulated. These elements 10 are defined by etching a pattern through a parent wafer 12 but leaving each element 10 connected to the wafer 12 by one or more relatively thin bridge sections 16. These bridges 16 have been designed to retain the full thickness of the parent wafer 12, and are necessary in order to have each element 10 remain in place so that further wafer 12 processing can be done in bulk. The use of bridges 16 avoids a handling problem that would occur with individual elements 10, as shown in FIG. 2.
Photolithographic processing of piezoelectric wafers 12 is known in the art and includes coating a piezoelectric wafer 12 with metal 24 and photoresist 26, respectively, as shown in FIG. 4. This is followed by developing the photoresist 26, etching the metal 24, and etching through the piezoelectric wafer 12 to define each element 10. Additional processing may include photoresist stripping, wafer cleaning and element 10 testing procedures which are much easier to accomplish when all the elements 10 are held in known positions on the parent wafer 12. However, after fully processing the wafer 12 it eventually becomes necessary to singulate the elements 10 from the parent wafer 12 in order to mount the elements 10 in individual packages. To assist in singulation, a relatively large relief area 20 (in FIG. 1), is etched through the parent wafer 12 near the separation edge 14 at the same time the element 10 is being defined, by etching through a remaining boundary 22 around the periphery of the element 10. This etching process leaves the element 10 connected to the parent wafer 12 by one or more bridges 16 which retain the full thickness of the parent wafer 12. The use of a relief area 20 near the separation edge 14 of the element 10 concentrates the breaking stresses of singulation at or near the bridges 16.
FIG. 2 shows a prior art singulated piezoelectric element 10. Singulation of the elements 10 from their parent wafer 12 by breaking the bridges 16 near the separation edges 14 can cause unpredictable damage. Many times, the body of the element 10 will fracture instead of the bridges 16. This type of breakage destroys the usability of the element 10, increasing scrap, lowering yields and raising costs. Even when the bridges 16 fracture as required, there occasionally remains irregular spurs 18 on the separation edge 14 of the piezoelectric element 10 where the bridges 16 did not break cleanly. Often, these irregular spurs 18 on the separation edge 14 are so pronounced that it is not possible to mount the element on that edge 14. Therefore, it is necessary to visually observe the quality of the edges of the piezoelectric element in order to mount the element on it's best edge, usually being the opposite edge 28. This type of visual inspection is a hindrance to automation.
FIG. 3 shows a cross-sectional view across the relief area 20 used in prior art etching of piezoelectric elements 10. The widths of the etched-through relief areas 20 and boundaries 22 are so large that anisotropic etching of the piezoelectric material and alignment of the top and bottom photolithographic patterning are essentially not a consideration in the prior art. The prior art assumes that all areas etched from both the top and bottom of a wafer 12 should always intersect (shown as item 20), to etch through the wafer 12.
Another consideration in the singulation of photolithographically produced blanks is that piezoelectric elements are very sensitive to their environment. Any particulates that contact the surface of the piezoelectric element can alter its frequency. Typically, an element is hermetically sealed under very controlled conditions. However, these sealing processes do not address particulate contamination that may occur during singulation. Previously, on photolithographically produced elements, singulation of the elements caused unpredictable fragmentation of the bridges (shown as 16 in FIG. 1). These bridges would fragment into pieces having a large variety of sizes. The smallest fragments adhere to the surface of the piezoelectric element by cohesive forces. The presence of this microscopic particulate matter on the surface of an element causes alterations of the frequency response of the element at varying input power levels. This phenomenon is known is "drive level dependence" or "starting resistance" in the art.
The most common method for eliminating this microscopic particulate is to add various cleaning stages to the process. Prior art cleaning can include mechanical brushing, wet processing, ultrasonic cleaner, plasma cleaner and the like. These solutions are problematic in that there is the potential of introducing more contamination than was originally present, unless these cleaning processes are very well controlled and monitored. The present invention solves this problem by minimizing the generation of particulate matter before potential problems occur. In addition, extra cleaning processes can be avoided, thus lowering costs.
A significant portion of the cost of a quartz blank is in processing, yield and labor. Scrap costs due to yield losses are to be minimized if at all possible. Yields losses result from breakage and contamination of blanks. Reducing inspection steps and operator involvement can save money and reduce the possibility for error. Improvements in automatability, yield, cost and quality can be achieved if an inherently clean and repeatable method for singulating uniform quartz blanks is used.
There is a need for an improved method for singulating quartz blanks, that: (i) is low cost; (ii) improves yields; (iii) minimizes the potential for damage to the piezoelectric element; (iv) reduces particulate contamination on the piezoelectric element due to ineffectual breakout of the piezoelectric element; (v) reduces processing; and (vi) produces substantially uniform blank dimensions, which can contribute to simplifying mounting and reducing the need for inspection.